The present invention relates to ion beam sputter deposition of multilayer films, particularly to controlling the thickness of the individual layers of the multilayer films, and more particularly to a method for controlling film thickness by keeping the total current supplied to both the beam and suppressor grids of an RF ion beam source constant by adjusting the plasma density (RF power).
Ion beam sputter deposition is used for a variety of applications, particularly in the semiconductor, magnetics and optical industries. Film thickness control is critical for many of these applications, such as the fabrication of masks for extreme ultraviolet (EUV) lithography. In the past, a great amount of effort has been directed to maintaining wavelength control in ion beam sputter deposition, particularly in blanks deposited in a mask blank tool. Typically, the film thickness has been controlled by supplying a constant beam current for a constant time. The current supplied to the beam grid is kept constant by changing the plasma density. Grid interception of the ion beam can change with time and makes regulating deposition rate in this manner unstable. Thus, there has been a need for controlling beam current whereby the current of the ion beam is maintained constant and thus the thickness of the films being deposited is maintained constant, and therefore the blank wavelength is constant.
The present invention provides a solution to this need by providing a method involving the measurement of the current to the suppressor and beam grids, and controlling the total current applied to both grids constant. Thus, using the method of this invention, the deposition rate of a three grid, RF ion beam source is controlled by keeping the total current supplied to the beam (inner) and suppressor (middle) grids constant by adjusting the plasma density (RF power).
It is an object of the present invention to provide for controlled thickness of a film deposited by ion beam sputtering.
A further object of the invention is to provide a method for measuring and controlling beam current in ion beam processing.
Another object of the invention is to provide film thickness control during ion beam sputter deposition by keeping constant the total current supplied to the beam and suppressor grids of an ion beam source.
Another object of the invention is to provide a method for controlling the deposition rate of an RF ion beam source by providing a constant current to both the beam grid and the suppressor grid by adjusting the plasma density of the source.
Other objects and advantages of the present invention will become apparent from the following description and accompanying drawings. The invention involves controlling film thickness by measuring and controlling the current to both the beam grid and the suppressor grid of an ion beam source. Since grid interception of an ion beam can change with time and thus change the rate of deposition, thereby varying the thickness of a film being deposited, by measuring the current of at least the suppressor grid as well as the beam grid, and maintaining the current of both grids constant, deposition rate and thus film thickness can be effectively controlled. The method of this invention, which involves calculating deposition rates based on the total of the suppressor and beam grid currents enables the control of deposited film thickness to about 0.1%. Such film control thickness is particularly critical for applications in the field of multilayer optics, semiconductors, and magnetics, since the method of this invention enables long term deposition stability.